Not Applicable
1. Field of the Invention
The present invention relates generally to golf clubs and, more particularly, to composite golf club shafts and methods of manufacturing the same.
2. Description of the Related Art
With the advent of composite golf club shafts, it has become much easier to tailor the design of a golf club to the needs of a particular player or particular shot. For example, for longer shots and lower numbered irons, or woods, it is often desirable to use a more flexible shaft. Whereas, for shorter shots and higher numbered irons, it is often more desirable to use a stiffer shaft. Such design goals may be achieved, for example, through the use of additional layers of composite fiber in shorter shafted clubs and through the use of fewer layers of fiber in longer clubs. Such design goals may also be achieved by varying the orientation of the layers of composite fiber that make up a shaft. For example, to add stiffness to a club shaft it may be desirable to utilize several layers of composite fiber that run parallel to the longitudinal axis of the shaft, whereas to enhance the flexibility of a shaft it may be desirable to utilize several layers of composite fiber which are offset to a substantial degree, for example, +/xe2x88x9245xc2x0 or more, from the longitudinal axis.
Those skilled in the art will appreciate that each layer of composite fiber may be formed using a xe2x80x9cpre-pregxe2x80x9d composite sheet, and that pre-preg composite sheets may be manufactured by pulling strands of fiber, for example, carbon or glass fiber, through a resin solution and allowing the resin to partially cure. Exemplary resins or xe2x80x9cbinding matricesxe2x80x9d may include, for example, thermosetting epoxy resins and thermoplastic resins. Alternatively, pre-preg sheets may be manufactured by pulling a fabric or weave of composite fabric through a resin solution and allowing the resin to partially cure. In either case, once the resin is partially cured or xe2x80x9cstaged,xe2x80x9d the resin holds the fibers together such that the fibers form a malleable sheet.
Similarly, selected regions of a club shaft may be reinforced through the provision of additional layers of composite fiber and by varying the direction of the composite fibers that may be found in a given layer. Indeed, it is quite common to provide additional layers of composite fiber in the tip region of a shaft to increase the torsional rigidity of the tip region and to insure that the tip region will not be damaged when a club head affixed thereto contacts a ball.
Finally, it has been found that by providing additional layers of composite fiber or by using pre-preg composite sheets having a weighting agent, such as iron, copper or tungsten powder, distributed therein, it is possible to adjust the overall weight, swing weight and balance point of a given shaft with a fairly reasonable degree of precision. Thus, it is possible conventionally to increase the overall weight of shorter club shafts through the use of additional fiber layers or through the use of weighted fiber sheets. Similarly, balance points and swing weights may be adjusted through the selective placement of additional fiber layers or weighted fiber sheets within a shaft.
There are, however, substantial drawbacks that are encountered when conventional techniques are used to adjust to any significant degree the overall weight, swing weight or balance point of a club shaft. For example, the addition of each layer of fiber alters to a significant degree the torsional and longitudinal flexibility characteristics of the shaft. The use of additional layers of fiber may also add substantially to the thickness of the walls of a shaft resulting in a dissimilarity in shaft wall thicknesses within a set. The reason for this is that many layers of fiber will generally be required to achieve a significant increase in shaft mass. It follows that, when conventional methods are used, it can be quite difficult to adjust the weight or distribution of mass within a golf club shaft without also varying to a significant degree the flexibility characteristics of the shaft.
The present invention is directed to methods of manufacturing golf club shafts, wherein the overall weight, swing weight and balance point of a club shaft may be adjusted or defined without simultaneously altering to any significant degree the flexibility characteristics of the shafts. The invention is also directed to club shafts manufactured in accordance with such methods.
Turning first to the structure of golf club shafts in accordance with the present invention, such shafts may be formed from and comprise a plurality of plies of pre-preg composite sheet and one or more plies of a loaded film. The loaded film may comprise a thin resin film that has a weighting agent, for example, copper or iron powder, evenly distributed therein.
Thus, in accordance with the present invention, the type, dimensions, location and orientation of the plies of pre-preg composite sheet within a golf club shaft may be used to define in part the flexibility characteristics of the shaft, while the dimensions and location of one or more plies of loaded film may be used to define and/or adjust in part the overall weight, swing weight and balance point of the shaft. It will also be noted that the shafts of a set of clubs in accordance with the present invention may differ in mass by as much as 80 grams, and possibly more, without incurring substantial differences in the thickness of the shaft wall, profiles or flexibility characteristics.
To manufacture a golf club shaft in accordance with the present invention, the following steps are preferably followed. The dimensions and relative positions of the plies of pre-preg composite fiber sheet and loaded film are determined, and a set of plies to be used in the shaft is prepared. A mandrel having predefined dimensions is selected and covered by a bladder manufactured, for example, from latex. The plies are then wound around the bladder covered mandrel in a predetermined manner, and the wrapped mandrel is placed in a mold. The mandrel may then be removed, leaving the bladder and surrounding plies in the mold. A source of pressurized gas may then be used to inflate the bladder and force the plies of pre-preg and loaded film against the walls of the mold, and the mold may be placed in an oven for a selected period of time, i.e., a time sufficient to allow proper curing of the resin comprising various plies. Thereafter, the bladder may be removed from the core of the shaft, and the shaft itself may be removed from the mold.
Those skilled in the art will recognize, of course, that numerous other methods may be employed to manufacture golf club shafts in accordance with the present invention and that the above-described method is but one example of those methods. For example, in alternative embodiments it may be desirable to leave the mandrel in the mold during the curing process, or it may be desirable to forgo applying internal pressure to the core of each shaft. In such an embodiment, a binding material such as cellophane or polypropylene tape may be wrapped around the outermost ply layer and used to hold the various plies against the mandrel during the curing process.
Having briefly described the present invention, the above and further objects, features and advantages thereof will be recognized by those skilled in the pertinent art from the following detailed description of the invention when taken in conjunction with the accompanying drawings.